In recent years, enhancement of the thermal efficiency of thermal power generation has been investigated as a potential energy conservation measure. In order to enhance the electric power generation efficiency of a power-generating gas turbine, increasing the gas inlet temperature has been shown to be effective, and in some cases this temperature is increased to approximately 1500° C. In order to realize a power generation plant that can be operated at this type of higher temperature, the stationary blades and moving blades that constitute the gas turbine, and the walls of the combustor and the like must be formed of heat-resistant members. However, even though the material used for the turbine blades is a heat-resistant metal, it is unable to withstand the types of high temperature mentioned above, and therefore a thermal barrier coating (TBC) is formed by using a deposition process such as thermal spraying to laminate a ceramic layer composed of an oxide ceramic onto the heat-resistant metal substrate, with a metal bonding layer disposed therebetween, thereby protecting the heat-resistant metal substrate from high temperatures. ZrO2-based materials are used for the ceramic layer, and yttria-stabilized zirconia (YSZ), which is ZrO2 that has been partially or totally stabilized by Y2O3, is often used because of its comparatively low thermal conductivity and comparatively high coefficient of thermal expansion compared with other ceramic materials.
Depending on the type of gas turbine, it is thought that the turbine inlet temperature may rise to a temperature exceeding 1500° C. In those cases where the moving blades and stationary blades and the like of a gas turbine are coated with a thermal barrier coating comprising a ceramic layer composed of the above-mentioned YSZ, there is a possibility that portions of the ceramic layer may detach during operation of the gas turbine under severe operating conditions exceeding 1500° C., resulting in a loss of heat resistance. Further, recent trends towards improved environmental friendliness are spurring the development of gas turbines having even higher thermal efficiency, and it is thought that turbine inlet temperatures may reach 1600° C. to 1700° C., with the surface temperature of the turbine blades reaching temperatures as high as 1300° C. Accordingly, even higher levels of heat resistance and thermal barrier properties are now being demanded of thermal barrier coatings.
The above-mentioned problem of detachment of ceramic layers composed of YSZ occurs because the crystal stability of YSZ is unsatisfactory under high-temperature conditions, and because the YSZ lacks satisfactory durability relative to large thermal stress. As a result, materials such as Yb2O3-doped ZrO2 (PTL 1), Dy2O3-doped ZrO2 (PTL 2), Er2O3-doped ZrO2 (PTL 3), and SmYbZr2O7 (PTL 4) have been developed as ceramic layers that exhibit excellent crystal stability under high-temperature conditions and superior thermal durability.
As disclosed in PTL 5, ceramic layers generally employ particles having an average particle size of 10 μm to 100 μm, and are typically deposited by a thermal spraying process.